The present invention relates to digital logic circuits, and particularly concerns a driver for repowering digital signals traveling in either direction along an electrical line.
Many digital-logic systems require drivers, buffers or amplifiers to repower signals on long or heavily-loaded lines. Frequently, as in bus-oriented memory or microprocessor systems, at least some of the lines carry signals in either direction at different times. Conventional bidirectional line drivers employ an external control signal to switch between two sets of unidirectional drivers connected in a back-to-back fashion; but some systems may not produce such a signal, or the difficulty and time delay in producing such a signal may be unacceptable. There are drivers which switch direction automatically, by sensing their own input-output (I/O) nodes, such as the circuit described by D. J. Johnston, "Redriver for Two-Way Bus", IBM Technical Disclosure Bulletin, August 1975, pages 663, 664. Conventional circuits of this type are inherently dependent upon circuit delays for proper operation. Some conditions of external circuit loading, or of manufacturing process variations, implementation technology, etc. can unintentionally vary the delay parameters so that a "glitch" (a spurious pulse) is produced on one of the I/O lines, and is then propagated to other circuits. The exact manner in which this fault occurs is described more fully below.